1. Field of the Invention
This invention relates to integrated circuit structures. More particularly, this invention relates to a modified multiple layer metal line for use with tungsten-filled vias in an integrated circuit structure.
2. Description of the Related Art
Metal lines, used in the construction of an integrated circuit structure to provide electrical communication between various active and passive devices in the circuitry of the integrated circuit structure, are usually constructed as multilayer structures. The center or core layer of the multilayer line is a metal layer selected for its high electrical conductivity, i.e., its low resistance. However, since the high conductivity core metal layer sometimes is reactive with other materials in the integrated circuit structure, i.e., with the tungsten used to fill vias electrically connected to the metal line, it is customary to place one or more protective layers of electrically conductive material over and under the core metal layer. Furthermore, to inhibit undesirable reflectance from the metal line (which can interfere with the photolithography used to form further structure over the metal layers which will be patterned into metal lines), it is also conventional to provide an anti-reflective coating (ARC) layer of electrically conductive material over such multilayer metal lines. However, such protective layers of electrically conductive material are basically chosen for their protective value, not their superior electrical conductivity. The resistance of such protective layers, while low enough to qualify the protective material as electrically conductive, is usually higher than the resistance of the metal core layer.
FIG. 1 shows a typical prior art composite or multilayer metal line structure 4 formed over a portion of an integrated circuit structure 2, such as, for example, a dielectric layer formed over a semiconductor substrate. Composite metal line structure 4 comprises a Ti/TiN or TiN protective lower layer 10; a central core metal layer 20, which may comprise any conventional highly conductive metal such as, for example, gold, aluminum, copper, or an aluminum-copper alloy which may optionally also contain silicon; and a top layer 30 of either Ti/TiN or TiN which doubles as both a protective layer for the underlying core metal layer and as an ARC layer.
Upper Ti/TiN or TiN layer 30 also acts as an etch stop when a further dielectric layer 40, e.g., a SiO.sub.2 layer, is formed over composite metal line structure 4, as shown in prior art FIG. 2, and a via 44 is etched through dielectric layer 40 down to (and, in this case for illustrative purposes, through) Ti/TiN or TiN layer 30. Because of this additional function of Ti/TiN or TiN layer 30 as an etch stop layer, it is usually constructed thicker than would be necessary for layer 30 to function merely as either an ARC layer or a protective layer to isolate the tungsten filler material eventually used to fill the via from core metal layer 20 in composite metal line structure 4.
For example, lower Ti/TiN or TiN layer 10, which only acts as a protective layer, usually ranges in thickness from about 5 nanometers (nm) to about 50 nm. Typically the thickness of layer 10 is about 25 nm. (250 Angstroms) In contrast, however, upper Ti/TiN or TiN layer 30 (which functions not only as a protective layer, but also as an ARC layer, and as an etch stop layer) usually ranges in thickness from at least 50 nm to about 100 nm, and typically the thickness of layer 30 is about 70 nm (700 Angstroms). That is, upper layer 30, to also function as an etch stop layer as well, must be substantially thicker than lower layer 10. However, since Ti/TiN or TiN comprises a high resistance electrically conductive material (relative to the resistance of core metal layer 20), it is also desirable to make layer 30 as thin as possible.
Furthermore, as shown in FIG. 2, it is customary, in the construction of a tungsten-filled via 44 in dielectric layer 40 over a composite metal line structure 4, to line not only the walls of via 44 with a material such as TiN, as shown at 46, to assure good adhesion of tungsten filler material 50 to the SiO.sub.2 walls of via 44, but also to line the bottom of via 44 with TiN as well, as shown at 48. The bottom liner 48 of TiN provides additional protection from a reaction between tungsten filler material 50 in via 44 and metal core layer 20 in composite metal line structure 4, just in case the etch step used to form via 44 in dielectric layer 40 penetrates through etch stop Ti/TiN or TiN layer 30 (as in the illustration of FIG. 2). This lining of the bottom of via 44 with a high resistance material like TiN adds yet further resistance to the structure, particularly when, as shown in FIG. 3, the etching of via 44 through dielectric layer 40 does stop (as intended) at the upper surface of Ti/TiN or TiN etch stop layer 30.
It would, therefore, be desirable to provide a composite metal line structure wherein the tungsten in a via subsequently constructed over the composite metal line structure would be isolated from the core metal material in the composite metal line structure, and ARC protection would be provided at the upper surface of the composite metal line, yet the need to line the bottom of the via with additional TiN, and the need to provide a thick upper layer of high resistance Ti/TiN or TiN in composite metal line structure 4 to act as an etch stop in the subsequent formation of vias, would all be eliminated.